Marking as resolved.
Here's a rundown of the findings, and the changes made
- Siteground's plugin was setup on the WordPress site, but in fact, was slowing down the page loading time. At various points, we were seeing up to 3.4 seconds !
- We took the decision to remove this, and replace with WP-Rocket, with @Justoverclock buying a valid license
- We configured and tuned all available settings
As a result of this interaction, the site loads in less that one second :O
We've combined CSS, JS, preloaded all font files, and enabled image lazy-loading. As this forum matures, the author is considering going down the WebP route for images to make the loading even faster.
So, if your current WordPress runs like a snail, and you'd prefer a Ferrari, let me know :)